The present invention relates to a fuel cell system and more particularly to a fuel cell system using a proton exchange membrane as an electrolyte.
A proton exchange membrane fuel cell comprises a proton exchange membrane (PEM) between two electrodes that is a cathode to which an oxidizing gas is supplied and an anode to which fuel gas is supplied. PEM acts as an electrolyte and transports therethrough hydrogen ions obtained at the anode of the fuel cell toward the cathode in the form of proton (H+). Each of the electrodes comprises a catalyst layer deposited on a porous base member through which the reactant gas is supplied. Mounted externally of each electrode is a separator or connector plate with grooves permitting the reactant gas to be introduced into the electrode at a constant flow rate. Excess gas which has not been consumed by the fuel cell reaction is exhausted to the open air through the grooved separator. The electricity generated by the energy conversion reaction at the anode is collected at the electrode porous base member and transported to the outside of the fuel cell system through the separator. In actual application, the system includes a plurality of fuel cells which are stacked in series with the separator being interposed between adjacent fuel cells.
Since the fuel cell generates heat in correspondence to the electric power generated, a fuel cell stack usually includes cooling plates between fuel cells at predetermined intervals. Each cooling plate has a passage of a cooling medium such as air and water to prevent overheating of fuel cells in operation.
Protons are hydrated in transfer through the PEM electrolyte, so that the PEM tends to become dehydrated as the fuel cell reaction proceeds. The PEM must always be properly humidified to prevent decrease of ion-conductivity and energy conversion efficiency. In the conventional designs, hydrogen gas is humidified by suitable means which, in turn, humidify the PEM when it is supplied to the anode.
Various attempts have also been proposed to humidify the air supplied to the cathode. Since the cathode of the fuel cell operates at 80xc2x0 C., for example, the air of a normal temperature should be preheated by a humidifier so that its saturated vapor becomes consistent with the ambient vapor condition of the cathode. Such a humidifier that is required to have both a water supplying function and an air preheating function can not be simple in construction.
In Japanese patent un-examined publication No. 7-14599, there is provided a water injection nozzle to inject a necessary quantity of water into an air introducing pipe through which air is supplied to the cathode of the PEM fuel cell. Since the nozzle is located upstream of a compressor, liquid water injected from the nozzle is evaporated by the heat generated by the compressor. Thus, the cathode is humidified by vapor, not by liquid water.
In the fuel cell system of Japanese patent un-examined publication No. 9-266004, a discharge gas from the anode containing hydrogen gas which has not been consumed during the anodic reaction is introduced into the cathode where the unconsumed hydrogen gas in the discharge gas is combusted with oxygen to generate water, which well humidifies the PEM electrolyte. In this system, there is no need to install a humidifier for humidifying the air supplied to the cathode.
During operation of the fuel cell system, electrons produced at the anode move to the cathode where they react with oxygen in the air or any other oxidizing gas supplied thereto to produce water. Accordingly, in accordance with the conventional practice in the art, there is a greater need to humidify hydrogen gas to be supplied to the anode, than at the cathode where supply of water is at least partially self-sustaining.
As a result of the inventors"" repeated tests and investigation, however, it has been found that water produced at the cathode permeates through the PEM electrolyte toward the anode, which makes it unnecessary to humidify hydrogen gas to be supplied to the anode. On the other hand, the quantity of water in the PEM electrolyte at the cathode tends to be decreased by the air flow to the cathode. Such finding is contradictory to the conventional understanding and has been first recognized by the present inventors.
It is an object of the present invention to provide a fuel cell system, based on the above-described finding, which is capable of maintaining a proton exchange membrane in a suitably moist condition.
Another object of the present invention is to provide a fuel cell system which is simple in construction, small in size, easy to install and, therefore, particularly suitable to be mounted on a vehicle.
Still another object of the present invention is to smoothly and effectively humidify an electrolyte membrane in a fuel cell system during the start-up operation of the system.
Accordingly, the present invention provides a fuel cell system in which water is supplied to the surface of the cathode, not in a vapor state, but in a liquid state. Thus, the fuel cell system of the present invention comprises one or more fuel cells each having an anode, a cathode and an electrolyte membrane interposed between the anode and the cathode; first gas supplying means for supplying a first gas including fuel gas to the anode; second gas supplying means for supplying a second gas including oxygen to the cathode; liquid water supplying means for supplying liquid water to the surface of the cathode; and control means for controlling operation of the second gas supplying means and the liquid water supplying means such that, when the system starts up, the cathode first receives supply of the second gas, followed by supply of the liquid water. Liquid water supplied onto the surface of the cathode may preferentially take latent heat from the air around the cathode to prevent water evaporation from the electrolyte membrane which, therefore, remains in a suitably and evenly moist condition. This contributes to improvement of capacity and durability of the fuel cell system. Supply of the liquid water is also effective to cool the cathode which would otherwise become overheated to an excessive temperature, which means that the temperature of the fuel cell of the present invention may be controlled without need to use cooling plates.
In accordance with the above aspect of the present invention, the liquid water supplying means may supply the liquid water continuously during start-up operation of the system. In a modified application, the liquid water supplying means supplies the liquid water for a predetermined period during start-up operation of the system. Accordingly, the electrolyte membrane, which could have become too dry to provide its original performance after a long time interval from the last operation of the fuel cell system, is readily humidified. The liquid water supplied by the liquid water supplying means is well dispersed onto the surface of the cathode by the flow of the first gas such as the air supplied by the second gas supplying means.
In accordance with another aspect of the present invention, there is provided a fuel cell system comprising one or more of fuel cells each having an anode, a cathode and an electrolyte membrane interposed between the anode and the cathode; first gas supplying means for supplying a first gas including fuel gas to the anode; second gas supplying means for supplying a second gas including oxygen to the cathode; liquid water supplying means for supplying liquid water onto the surface of the cathode; sensor means for detecting water supply capacity of the liquid water supplying means at least when the system starts up; and control means for discontinuing or interrupting operation of the system when the sensor means detects that water supply capacity of the liquid water supplying means has decreased to below a predetermined minimum level. In a preferred embodiment, the liquid water supplying means includes a water tank, the sensor means comprises a water level sensor that detects a water level in the water tank, and the control means discontinues or interrupts the system operation when the water level in the water tank detected by the water level sensor is decreased to below a predetermined minimum water level. In another preferred embodiment, when the water supply capacity of the liquid water supplying means is found to be below a predetermined minimum level, the control means outputs a signal to alarm means that outputs some visual or audible alarm sign to urge an operator of the system to supplement liquid water to the liquid water supplying means.
In accordance with still another aspect of the present invention, there is provided a fuel cell system comprising one or more fuel cells each having an anode, a cathode and an electrolyte membrane interposed between the anode and the cathode; fuel gas supplying means for supplying a fuel gas to the anode; fuel gas discharge means for discharging the fuel gas from the anode; and control means for controlling the fuel gas supplying means and fuel gas discharge means such that, when the system starts up, the fuel gas discharge means is opened for a predetermined period and then closed. The gas supplying means starts continuous supply of the fuel gas after opening the fuel gas discharge means. When the fuel gas supplying means comprises a hydrogen storing alloy such as LaNi5, TiFe, ZrMn2, Mg2Ni which produces hydrogen gas when heated, the maximum internal pressure in the fuel gas supplying means may exceed the pressure-proof capacity of the fuel cell. For this reason, the fuel gas supply passage from the hydrogen storing alloy tank to the anode of the fuel cell includes a valve that is opened under control to reduce the hydrogen gas pressure. However, when the tank is first opened, irrespective of operation of the valve, an extraordinarily high pressure could be applied to the fuel cell, which could cause serious damage to the electrolyte membrane. This is prevented by the above aspect of the present invention in which, even if an extraordinarily high pressure is applied to the fuel cell at the time when the tank is first opened, it is smoothly discharged through the fuel gas discharge means to the open air.
In the above aspect of the present invention, it is preferable to provide liquid water supplying means for supplying liquid water to the surface of the cathode, which is controlled by the control means to start supply of the liquid water to the surface of the cathode before opening the fuel gas discharge means.
In accordance with still another aspect of the present invention, there is provided a fuel cell system including one or more fuel cells each having an anode, a cathode and an electrolyte membrane interposed between the anode and the cathode; first gas supplying means for supplying a first gas including fuel gas to the anode; second gas supplying means for supplying a second gas including oxygen to the cathode; liquid water supplying means for supplying liquid water to the surface of the cathode; sensor means for detecting a temperature of gas discharged from the fuel cells; and control means responsive to detection by the sensor means to control operation of the liquid water supplying means such that the liquid water is supplied to the surface of the cathode only when the sensor means detects that the discharge gas from the fuel cells has a temperature below a first predetermined temperature while the system is in operation. This minimizes energy consumption by the liquid water supplying means and improves energy efficiency in the overall fuel cell system. In a preferred embodiment, the liquid water supplying means operates to continuously supply liquid water to the surface of the cathode when the sensor means detects that the discharge gas from the fuel cells has a temperature above the first predetermined temperature but still below a second predetermined temperature. In a more preferred embodiment, the control means interrupts operation of the system when the sensor means detects that the discharge gas from the fuel cells has a temperature above the second predetermined temperature.
In accordance with still another aspect of the present invention, there is provided a fuel cell system comprising one or more of fuel cells each having an anode, a cathode and an electrolyte membrane interposed between the anode and the cathode, including a fuel gas passage extending through the anode; fuel gas supplying means for supplying a fuel gas to the fuel gas passage; interruption means for closing the fuel gas passage while it is filled with the fuel gas supplied by the fuel gas supplying means; and pressure loss sensor means for detecting a pressure loss in the fuel gas passage. This aspect of the present invention is based on finding that there is a tendency of fuel gas to leak through the electrolyte membrane as it deteriorates with time. Accordingly. when an excessive degree of pressure loss in the fuel gas passage is detected by the sensor means, it may be inferred that the electrolyte membrane has deteriorated to the extent that it should be replaced with a fresh one.
In the preferred operation, the output of the fuel cells is connected to an external load when the output voltage detected by an output voltage sensor means satisfies predetermined conditions. This prevents an excessive load from being applied to the fuel cells and, therefore, prevents deterioration of the fuel cells and damage to the electrolyte membrane. It is preferable that the predetermined conditions include not only the total output voltage of the fuel cell stack but also the output voltage of a single unit fuel cell or a couple or series of fuel cells. By way of example, the fuel cells are connected when the output voltage of a single unit fuel cell or a predetermined series of fuel cells reaches a first predetermined level (8V, for example) and the total output voltage of the fuel cell stack reaches a second predetermined level (38V, for example).
In accordance with still another aspect of the present invention, there is provided a fuel cell system comprising one or more fuel cells each having an anode, a cathode and an electrolyte membrane interposed between the anode and the cathode; fuel gas supplying means for supplying a fuel gas to the anode; fuel gas discharge means for discharging the fuel gas from the anode; and stop control means for interrupting output from the fuel cells, then closing the fuel gas supplying means and then closing the fuel gas discharge means. This minimizes the quantity of the fuel gas still remaining after the fuel cell operation is discontinued and prevents excessive post fuel cell reaction which would deteriorate the fuel cells. More preferably, supply of air and water to the cathode is interrupted after the fuel gas discharge means is closed, which suppresses heat generation by post fuel cell reaction.
In accordance with still another aspect of the present invention, there is provided a fuel cell system including one or more of fuel cells each having an anode, a cathode and an electrolyte membrane interposed between said anode and said cathode; first gas supplying means for supplying a first gas including a fuel gas to said anode; fuel gas discharge means for discharging said fuel gas from said anode; second gas supplying means for supplying a second gas including oxygen to said cathode; liquid water supplying means for supplying liquid water to the surface of said cathode; and control means for controlling operation of said system such that, when said system is to be stopped, said first gas supplying means, then said fuel gas discharge means and then said liquid water supplying means are stopped in this order.